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Full text of "Text Book Of Mechanical Engineering"

Hydraulic Cranes.

741

whole apparatus is fixed to the crane by feet j j. A wire rope or
chain being attached to the eyebolt K, and carried round the
pullies E1D1E2D2E3, leaves D3, by w, to the load or slewing:
wheel, as desired. Examining by the pulley principle Fig. 439,
p. 483, the mechanical advantage will be inversely as the number
of cords or chains at L L, Fig. 729, p being now the greater, and
w the lesser force. Neglecting friction,

W___!___

P "~ no. of cords

Mech. Adv. = 

And allowing for all resistances, 

P

W

no. of cords

where  efficiency 77 varies  with  the number of pullies,   by the
following table, found from practice :

VALUES OF ?? FOR HYDRAULIC CRANES.

No. OF PULLIES.
	o
	2
	4
	6
	8
	10
	12
	14
	16

77 =
	87
	8
	76
	72
	-67
	63
	"59
	'54
	'5

and the greater tension, at tail end, equals P  (no. of cords x 17)*
Thus, a heavy pressure with slow speed has lifted a smaller
load at greater speed, the distance between pulley centres having
been increased.

In order that the ram shall finish its stroke quietly, automatic
cut-off gear is supplied. Valve H being opened to pressure by
raising rod N fully, the ram, ascending, strikes a tappet R by means
of the projection Q, when the stroke is nearly complete, thus
causing lever M to be pulled over to position s, and closing the
valve. A further movement of M to position T opens H to
exhaust, and the ram descends by the pull of the load,

Reference may now be made to Plates XV. and XVLy
shewing various hydraulic cranes. That on the left in Plate XV.
is the best example of pulley gear. Thus, cylinder D is for
lifting, E for traversing, and c for slewing, all worked from valves
at s.